Rhodium is the hardest, whitest and most chemically stable of the platinum-group of metals, but it is also one of the most expensive. Rhodium is used as a thick coating for engineering use for various applications involving repetitive wear and to protect electrical and electronic components from atmospheric corrosion, particularly at high temperatures. Particular examples of the use of rhodium as a coating are slip-rings and switches in tele-communication equipment and high speed computer switches and sliding contacts. Additionally rhodium is also used at a thickness of from 0.05 to 2 .mu.m on silverware and jewellery, at a thickness of from 1.25 to 6.25 .mu.m on reflector and searchlight surfaces and at a thickness of from 0.05 to 25 .mu.m on electrical contacts. In general the thicker the deposit the better the protection but it is difficult to deposit, a thick layer of rhodium without cracking due to build-up of internal stresses. With methods of coating used at the present time, the conditions of plating have to be carefully controlled to prevent cracking and because of this, the uses of rhodium as a coating are not as widespread as for other platinum-group metals. Processes that use special additives such as selenium and magnesium have been proposed. Processes have been proposed by A. E. Yaniv [Plating, 54,721 (1967)] that use particular apparatus. Processes have also been proposed that use high concentration baths of rhodium sulphamate or rhodium sulphate. However it is difficult to obtain crack-free coatings of rhodium at thicknesses greater than 2.5 um using direct current electroplating methods.
It is an object of the present invention to provide a process of producing crack-free rhodium coatings and rhodium sheets or foils, in particular rhodium sheets or foils having a thickness of from 10 .mu.m to 200 .mu.m.